Instrument Transformers Technical Information and Application Guide
Table of Contents Technology Review An introduction to instrument transformer fundamentals, a discussion on power quality, and a detailed explanation of design considerations to assist in selecting an instrument transformer. 3 Product Description A general overview of ABB s current and voltage transformers. This section is complemented with photographs and describes the specific classes and uses for instrument transformers. 37
Introduction 4 Types of Construction 6 Performance Characteristics 8 Accuracy Classifications for Metering 12 Burdens 13 Current Transformers: Accuracy Classes for Relaying 15 Insulation Systems 16 Voltage Ratings 18 Power Quality 22 Service Conditions 24 Auxiliary Current Transformers 31 Fusing Voltage Transformers 32 New Design Technologies 33 3
Introduction Instrument transformers (ITs) are designed to transform voltage or current from the high values in the transmission and distribution systems to the low values that can be utilized by low voltage metering devices. There are three primary applications for which ITs are used: metering (for energy billing and transaction purposes); protection control (for system protection and protective relaying purposes); and load survey (for economic management of industrial loads). Depending on the requirements for those applications, the IT design and construction can be quite different. Generally, the metering ITs require high accuracy in the range of normal operating voltage and current. Protection ITs require linearity in a wide range of voltages and currents. During a disturbance, such as system fault or overvoltage transients, the output of the IT is used by a protective relay to initiate an appropriate action (open or close a breaker, reconfigure the system, etc.) to mitigate the disturbance and protect the rest of the power system. Instrument transformers are the most common and economic way to detect a disturbance. Typical output levels of instrument transformers are 1-5 amperes and 115-120 volts for CTs and VTs, respectively. There are several classes of accuracy for instrument transformers defined by the IEEE, CSA, IEC, and ANSI standards. Figure 1 presents a conceptual design of CTs and VTs. Figure 2 shows how the polarity markers are used to keep the direction of current flow in the meters exactly the same, as if the primary circuit was carried through the meters. Grounding of the secondary circuit is most important, but in complicated threephase connections, the best point to ground is not always easily determined. 4
Figure 1: Current and Voltage Transformer Symbols and Simplified Concepts Symbol of a Current Transformer Conceptual picture of a Current Transformer I 1 I 1 I 2 I 2 Symbol of a Voltage Transformer Conceptual picture of a Voltage Transformer V 1 V 1 V 2 V 2 Figure 2: Instrument Transformer Connections Current Transformer H1 H2 Lines in which either current or voltage is too high to permit running the lines directly to the meter or relay H1 H2 X1 X2 H1 H2 X1 X2 Voltage Transformer Transformer or Power Sub-Station X1 X2 Ammeter A R Current Operated Relay V R Volt Meter Voltage Operated Relay W + - + - R + + - - Watt Meter Current-Voltage Relay A R + - + - Current-Current Relay A. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the meter or relay. The voltage transformer is designed to connect in parallel with the line to transform the line voltage to 115 or 120 volts suitable for the meter or relay. To keep the voltage at the meters and relays at a safe value, the secondary circuit must be grounded. B. The polarity markers indicate the relative instantaneous directions of current in the windings. The polarity, or instantaneous direction of current, is of no significant difference for current-operated or voltage-operated devices. Correct operation of current-current, voltage-voltage, or current-voltage devices usually depends on the relative instantaneous directions. B 5
Types of Construction The principal forms of construction used for instrument transformers, together with standard symbols according to IEEE Standard C57.13, are shown in Figure 3. Figure 3: Types of Instrument Transformer Construction Simple Basic Forms H1 X1 H1 H2 Primary Secondary X1 X2 CORE H2 X2 Current Transformer Current Example 5.5 Amperes Ratio 1:1 Voltage Transformer Voltage Example Primary 7200 Volts Ratio 60:1 or 7200:120 Volts Construction Types Window-type Bar-type Wound X2 X1 X2 X1 X2 X1 H1 H1 H1 Secondary Types Dual Ratio Example Multi-Ratio Example H1 X1 X2 H2 X3 200/400:5 Amperes 200 Amperes in H1-H2 will produce 5 Amperes in X2-X3 * 400 Amperes in H1-H2 will produce 5 Amperes in X1-X3 * * See nameplate for actual connections 600:5 Multi-Ratio Secondary Winding 20 10 50 40 X1 X2 X3 X4 X5 Primary Ampreres 50 100 150 200 250 300 400 450 500 600 600:5 Multi-Ratio Secondary terminals to get 5 Amps X2-X3 X1-X2 X1-X3 X4-X5 X3-X4 X2-X4 X1-X4 X3-X5 X2-X5 X1-X5 6
Materials Used in Construction Butyl Rubbers Cycloaliphatic Epoxies In the 1970 s, the insulating medium for the higher voltage (5-34.5 kv) units was butyl rubber. The material itself is excellent, but the pressures and temperatures necessary to use it as a dielectric were not conducive to the exacting clearances and geometries inside a voltage or current transformer. Without excess bracing, the core/coil assemblies would shift during molding and fail BIL testing. With enough bracing, the material flow inside the unit was restricted, increasing the possibility of voids. For these reasons, another dielectric insulating material was sought. Butyl rubber is still used by some manufacturers. Cycloaliphatic epoxy (CEP) was first introduced in outdoor insulators in the late 1970's due to its very good resistance to humidity, ultraviolet (UV) radiation, outdoor pollutants, and chemicals. Its outstanding mechanical strength and dielectric properties were also highly desirable. Aromatic Polyurethanes Hydrophobic Cycloaliphatic Epoxy Aromatic polyurethane (PUR) elastomers are another cost-effective insulation for medium voltage electrical equipment. There are approximately fourteen general types of PUR that are successfully commercialized for a wide variety of applications, including instrument transformers. Thirteen of these types are called conventional rubber. That means they are mixed, milled, and molded by techniques which have been in use by the rubber industry since the 1920 s. Polyurethane rubber raw materials are liquid, which permits them to be pumped, metered, mixed, and dispensed by machines under very precise control of temperature and ingredient proportions. The liquid mixture enters the mold at vacuum pressure and is cured at slightly elevated temperatures. This unique characteristic allows molding of large parts which are completely uniform throughout. When compared to high pressure-molded butyl rubber and vacuum cast epoxies, PUR in general has the most forgiving process. The fully cured PUR elastomers possess a desirable balance of ease of manufacturing (via vacuum casting), mechanical toughness (it is, after all, a rubber) and very good electrical properties. In the early 2000's, a global epoxy resin supplier introduced a hydrophobic version of CEP called Hydrophobic Cycloaliphatic Epoxy (HCEP) to the market. HCEP is formulated to sustain surface hydrophobicity better than its CEP counterpart upon prolonged exposure to aggressive outdoor environments without sacrificing other desirable chemical and mechanical properties. A hydrophobic insulation surface is desirable for outdoor applications because it prevents water from developing completely wetted, resistive conductive surfaces. Leakage currents are therefore reduced, which helps to reduce the flashover risk. The result is enhanced reliability. Furthermore, less discharge activity means less attack and therefore less surface erosion, which extends the transformer s life. 7
Performance Characteristics Equivalent Circuits The specific performance characteristics of instrument transformers are easily determined from the equivalent circuit. Figure 4 works well for most instrument transformers. For current transformers, the value of the reactance X is determined in a special way so that it represents the leakage flux. The flux flows in the part of the core represented by the left-hand exciting branch of the equivalent circuit shown in Figure 4. An additional winding (or windings) placed over the outer leg(s) of the core and connected back in parallel with the secondary winding, as shown in Figure 4, can keep the leakage flux out of the core. The leakage reactance is then effectively connected ahead of both exciting branches as shown in Figure 4. This difference is important for current transformers because leakage flux in the core affects current ratio. It also improves the performance of current transformers and subjects their performance to simple calculation. Voltage transformers are designed so the through impedance (R S,R,X P, and X) is as low as possible, while current transformers are designed so the excitation impedance (Z o and Z i ) is as high as possible. Neither transformer is very good at performing the function of the other. Figure 4: Equivalent Circuits A Resistance Primary Coil Leakage flux not reaching the core Primary winding Secondary winding Working flux Leakage flux which flows in the core represented by reactance "X" Core R p Internal Reactances X p X Secondary Resistance Coil R s C Auxiliary winding on outer leg of core connected in parallel R p X p X R s A. A typical transformer and its equivalent circuit. The leakage flux is shown entering the outer part of the core and is represented by reactance X. The reactance develops voltage applied to the exciting branch Z o, which represents the outer side of the core. The series impedance, R P + R S + j (X P + X), is responsible for the loss of voltage in transformation. The voltage transformers are carefully designed to keep this impedance as low as possible. The loss of current in transformation is due to current by-passed by the exciting branches, Z o and Z i. Current transformers are specially designed to keep these by-pass exciting impedances as high as possible. Input Perfect or Ideal Transformer B Core Z o Z i Output Exciting Impedance Leakage flux which flows in the core D Z o Z i B. A common construction of HV or EHV current transformer. Leakage flux enters the core even though the winding is uniformly wound over a ring core. The equivalent circuit is the same as for Figure A. C. A construction used in HV or EHV current transformers. The parallel auxiliary winding effectively keeps the leakage flux out of the core so that the leakage reactance in the equivalent circuit is effectively ahead of the exciting branches. This simplifies the calculation of the current by-passed through Z o and Z i. Leakage flux not reaching the core Secondary winding Primary winding enclosing one side of the core Returning primary circuit far enough away that leakage from it is negligible D. A typical bushing current transformer. This resembles the transformer in B but has only negligible leakage flux in the core because the return conductor is far away. This transformer still has a good deal of leakage reactance, but the leakage flux does not enter the core in significant amount. The reactance is ahead of the by-pass branches Z o and Z i so that the performance as a current transformer can be easily calculated. 8
The value of through impedance is constant, but the value of excitation impedance is variable. The exciting impedances representing the exciting currents for the two parts of the core depend on the voltage applied to them, the current flowing in them, or flux density in the core. The easiest way to understand Z o and Z i, which are of primary importance in current transformers, is to draw saturation curves showing how the current flowing into the exciting branch varies with the voltage applied, as shown in Figure 5. The curve is usually plotted for the combination of Z o and Z i in parallel. Voltage transformers are designed such that the operating point on the saturation curve, as shown in Figure 5, is typically at a relatively high voltage. This is subject to the limitation that this point must not be so high that the exciting current itself is excessive. Voltage transformers are designed to work without excessive exciting current up to 110% of rated voltage. The IEEE standard for performance requires good performance also at 90% voltage. Figure 5 shows that the exciting current will not reach a higher per unit value, with consequent increase of voltage loss from exciting current, at voltages over the range of 5% to 110%. Figure 5: Typical Transformer Saturation Curve 110 100 Voltage Applied to Terminals % 50 20 10 5 2 1 Current transformers normally operate at rated current in this range 0 1 2 5 10 20 50 100 300 Exciting Current % 45 line representing constant exciting reactance constant per unit exciting current Voltage transformers are designed such that the operating point on the saturation curve is typically at a relatively high voltage. The voltage must not be so high that the exciting current itself becomes too high. This would cause a voltage drop in the primary impedance that would bring about an excessive error in ratio of phase angle. IEEE C57.13 requires performance standards to be met at 110% of rated voltage. The curve shows the per unit exciting current, below which the error due to voltage drop caused by the exciting current itself will equal that at 110% rated voltage. Performance at voltages down to 5% is not significantly different at the same burden connected to the transformer secondary. The error limits required by C57.13 apply not only at a given burden but at zero burden. The current transformer, on the other hand, is designed to operate at the low range of the curve (see region marked on the curve) so that the exciting current by-pass will be as low as feasible. The curve shows that as the voltage is reduced, the exciting current is not reduced in proportion. This means, in a current transformer, that as the primary and secondary current decrease, the by-pass current which causes the error actually increases in percentage. The errors in current transformation typically increase at the lower currents. 9
Ratio Error and Phase Angle Ideal transformers induce the same voltage per turn in the secondary winding as that applied to the primary voltage transformers. They also produce the same ampere-turns in the secondary as circulated in the primary current transformers, to deliver any desired ratio of primary to secondary voltage or current. In the actual transformer shown in Figure 4, the secondary current output is deficient by the amount of current bypassed by the exciting branches, Z o and Z i, and the secondary output voltage is deficient by the voltage drop in the transformer through impedance. The transformer nameplates show a marked ratio, usually an even number, such as 20 to 1. The actual ratio of primary to secondary quantity may be slightly higher or lower than the marked value by an amount 1 called ratio error, which is defined in IEEE C57.13 as ratio correction factor (RCF). For instance, if the actual ratio is 20.2 to 1, then the RCF is 1.01 and the ratio error is 1%. The secondary output may be slightly out of phase with the primary input. This error is called phase angle (usually measured in minutes) and is designated as positive if the secondary output leads the primary input. 1. The ratio of turns is usually adjusted to compensate for the losses in the transformer. Because of this compensation, the actual secondary output can be higher than would be expected from the marked ratio. The Effect of Ratio Error and Phase Angle on Wattmeter Readings If the ratio correction factor exceeds 1.0, the meters will read low and the readings should be multiplied by the correction factor. The effect of phase angle, however, is not as obvious. The transformer correction factor (TCF) determines reasonable limits for RCF and phase angle. This depends on both RCF and phase angle and may be used to correct the reading of a wattmeter. TCF is based on the fact that if the power factor of a metered power load is 60% lagging (represents the usual minimum power factor of actual power loads being metered by watt hour meters), 2.6 minutes of phase angle in the current (or voltage) transformer output will cause 0.1% error in the wattmeter reading. 10
Voltage Transformers The RCF and phase angle must be determined at the specific burden involved. However, as with an actual measurement, these values apply to the secondary voltage at the transformer terminals. If the leads from the transformer to the burden are very long, they may have sufficient impedance to introduce additional voltage drop and error. The voltage drop in the leads can be calculated from the current drawn from the transformer and the impedance of the leads. If it is of appreciable magnitude in percent of secondary voltage, the addition to ratio and phase angle error may be calculated according to the vector diagram in Figure 6 and the formulas: Percent Ratio will be increased by: I + X S ( RL cos 0 L sin 0 ) x 100 E S Add this amount to the percent ratio of the transformer to get the actual percent ratio of primary to burden voltage. The Phase Angle will be increased by: I - X S ( RL sin 0 L cos 0 ) x 3438 Minutes E S Add this amount to the phase angle of the transformer (algebraically) to get the actual phase difference between primary and burden voltages. Figure 6: Effect of Leads in Voltage Transformers R L X L R s E s E p E s I s E Instrument I s 0 E Instrument I s R L I s X L Voltage Transformer R L and X L represent the resistance and reactance in the leads. Method for calculating impedance drops in the leads and resulting ratio and phase angle errors. Current Transformers The ratio correction factor and phase angle must be determined at the specific burden and current involved. Current transformer characteristics at special burdens can only be determined by an actual measurement. The test burden must duplicate the actual burden, including the secondary leads. The secondary terminal voltage and power factor must be identical to that of the installation. In addition, these measurements need to be made and applied at the actual service currents. 11
Accuracy Classifications for Metering Figures 7 and 8 provide an explanation of accuracy classes for current and voltage transformers. These figures show the Accuracy Classes as adopted by IEEE, as well as the special limitations which apply to current and voltage transformers. IEEE C57.13 has recognized 0.3% as a reasonable error limit and has designated this as Accuracy Class 0.3. Figure 7: The Basic 0.3 Class Parallelogram for Current Transformers Figure 8: The Basic 0.3 Class Parallelogram for Voltage Transformers RCF RCF 1.003 1.003 1.002 B B 1.002 1.001 1.001 15 Minus 10 5 5 10 15 Plus Phase Angle Minutes 15 10 5 5 10 15 Minus Plus Phase Angle Minutes 0.999 A 0.999 0.998 A 0.998 0.997 15.6 = 0.6 x 26 0.997 15.6 = 0.6 x 26 The Figure 7 parallelogram outlines the area in which the measurements of RCF and Phase Angle at 100% current must plot to designate the transformer accuracy as 0.3 Class with TCF within the limits of 0.997 and 1.003. For example, if the RCF at a given burden at 100% current is 0.998 (99.8% Ratio) and the Phase Angle is 3.5 minutes, point A is seen to fall outside the parallelogram. Another example: RCF = 1.002, Phase Angle 5 minutes, representing greater absolute error, point B is now inside the parallelogram and meets the required limits for 0.3 Accuracy Class. In the second case, TCF is less than 1.003 because the effect of phase angle on the wattmeter compensates for the error in ratio. IEEE C57.13 recognizes that current transformers naturally have greater errors at lower currents, and that error at low current does not usually represent significant error in total registration of kilowatt hours. This permits twice the error at 10% that is permitted at 100% current. The error at the maximum current permitted by the Thermal Rating Factor of the transformer (a multiplier of 1.5 or 4.0 applied to many transformers) is limited to the same value at 100% current. Other Accuracy Classes: In addition to the 0.3 Class, C57.13 recognizes the 0.6 and 1.2 Classes in which the permissible errors are twice as great (0.6%) and twice again (1.2%) as compared to the 0.3 Class. Any one of these classes may be selected for specification by the user depending on whether 0.3%, 0.6%, or 1.2% seems reasonable for a given application. The Figure 8 parallelogram outlines an area in which the measurements of RCF and Phase Angle at 100% (also at 110%) voltage must plot to designate the transformer accuracy as 0.3 Class with TCF within the limits of 0.997 and 1.003. For example, if the measured RCF at a given burden is 0.999 and the phase angle is -8 minutes, point A is seen to fall outside the parallelogram. Another example: RCF = 1.002, Phase angle is -10 minutes, both representing greater absolute errors, but the point B is now inside the parallelogram and meets the required limits for the 0.3 Class. In the second case, the TCF is less than 1.003 because the effect of phase angle on the wattmeter compensates for the phase angle. The reason for the reversed appearance of Figure 8 compared to Figure 7 is that phase angle in the current transformer brings the secondary current more nearly in phase with the load voltage, increasing the wattmeter reading. In the voltage transformer, the effect is just the opposite. C57.13 requires that the limits also be met at 90% voltage; in reality, the performance at voltages down to 5% are not significantly different at the same burden connected to the transformer secondary. The error limits required by C57.13 apply not only at a given burden, but also at zero burden. Other Accuracy Classes: In addition to the 0.3 Class, C57.13 recognizes the 0.6 and 1.2 Classes in which the permissible errors are twice as great (0.6%) and twice again (1.2%) as compared to the 0.3 Class. Any one of these classes may be selected for specification by the user depending on whether 0.3%, 0.6%, or 1.2% seems reasonable for a given application. 12
Burdens The errors in ratio and phase angle depend on the impedance connected to the secondary of the transformer. This impedance is commonly referred to as burden. The calculations required for determining the performance of a transformer when different burdens are applied are beyond the scope of this discussion. Therefore, the standard burdens as outlined in IEEE C57.13 are used to represent typical service conditions. Each transformer is rated according to its performance at these standard burdens. Standard Burdens for Current Transformers Many current transformers supply only a limited number of watthour meter elements with a limited number of runs. For metering and relaying applications, IEEE C57.13 has established the standard burdens as given in Figure 9. Figure 9: Standard Burdens for Current Transformers Standard Burdens for Current Transformers with 5 A Secondaries * Burden Designation + Resistance (Ω) Inductance (mh) Impedance (Ω) Volt Amperes (at 5 A) Power Factor Metering Burdens B-0.1 0.09 0.116 0.1 2.5 0.9 B-0.2 0.18 0.232 0.2 5.0 0.9 B-0.5 0.45 0.58 0.5 12.5 0.9 B-0.9 0.81 1.04 0.9 22.5 0.9 B-1.8 1.62 2.08 1.8 45.0 0.9 Relaying Burdens B-1 0.50 2.3 1.0 25.0 0.5 B-2 1.00 4.6 2.0 50.0 0.5 B-4 2.00 9.2 4.0 100.0 0.5 B-8 4.00 18.4 8.0 200.0 0.5 * If a current transformer is rated at other than 5 A, ohmic burdens for specification and rating may be derived by multiplying the resistance and inductance of the table by [5/(ampere rating)] 2 the VA at rated current and the power factor remaining the same. + These standard burden designations have no significance at frequencies other than 60 Hz. Actual Burdens for Current Transformers Actual devices connected to instrument transformers often include an inductor with an iron core, which usually means that the inductance is not constant but varies during the cycle, and varies differently with different currents. Exact analysis of current transformer performance with such devices is difficult. Fortunately, the impedances of most instruments and meters are sufficiently constant that no appreciable error is introduced by considering them to be constant. Many electro-mechanical relays, however, have variable impedance. Analysis of the transformer performance is usually based on an equivalent value at normal current. This can be justified on the basis that the burden at higher current is usually less and thus the current transformer will perform better than expected from the equivalent burden. 13
Some relays operate from two or more sources of current: differential (current-current), or power or impedance-measuring (current-voltage) relays. If the two circuits are magnetically coupled by the relay, the burden on one source is affected by the current in the other source, and vice versa. Most of the two-source relays act by balance-beam or other mechanical coupling, so that the burdens are fixed. Standard Burdens for Voltage Transformers The standard burdens to be used for testing and comparing voltage transformers are rated at 120 volts and at 69.3 volts. IEEE C57.13 specifies that the 120 volt-rated burden will be used for any transformer with the secondary voltage in the range of 115 to 120 volts, while the 69.3 volt burden will be used for any transformer with the secondary voltage in the range of 65 to 72 volts. This means that the actual volt amperes in the burden in a given test may be different than the nominal value of the burden in volt amperes. For instance, if the standard burden is 25 volt amperes, the actual burden when it is used for testing a transformer with 115 volt secondary is (115/120) 2 or.918 times the nominal value of 25. The burdens rated 69.3 volts have an impedance only one-third of that of burdens rated 120 volts and they should not be used in testing or rating transformers rated at 115 to 120 volts. Transformers rated at 115 or 120 volts should be treated as 115 or 120 volt transformers, and if they are actually used at reduced voltage, the performance will not be different if the 120 volt burden is used as a basis for performance. This is because the performance of a transformer down to voltages of about 5% of its rating is not significantly different from the performance at 100% voltage. Refer to Figure 10 for standard burdens for voltage transformers as outlined in IEEE C57.13. Figure 10: Standard Burdens for Voltage Transformers Standard Burdens for Voltage Transformers Metering Burdens Burden Designation Volt Amperes Power Factor Burden Impedance 120 V Burden 69.3 V Burden W 12.5 0.10 1152 384 X 25 0.70 576 192 M 35 0.20 411 137 Y 75 0.85 192 64 Z 200 0.85 72 24 ZZ 400 0.85 36 12 14
Current Transformers: Accuracy Classes for Relaying Relaying accuracy classes for CTs are defined with a C or a T classification. C indicates that the transformer ratio can be calculated. These are transformers which are constructed so that the effect of leakage fluxes on its performance are negligible. T indicates the transformer where the leakage flux has an appreciable effect on the ratio. Since the calculation of the excitation current by-passed is a tedious process, the performance of the transformer can only be determined by test. The basis for classification of performance for relaying is an error limit of 10% at any current from 1.0 to 20 times normal. The accuracy class is the description of how much voltage the transformer can supply to the output circuit (burden), without the CT core going into saturation. For example, a transformer that can supply a 2 ohm output circuit (burden) at 100 A [20 times normal current (5 A)] or 200 V, without saturating the core and within a 10% error limit, is classified as 200 accuracy class. Refer to Figure 11. Standard accuracy classes, which may be assigned for a relaying current transformer, are 50, 100, 200, and 800. If a C200 transformer can supply 100 A secondary output at exactly 10% error into a 2 ohm burden, then the exciting branch is not over 10 amperes. If the current is lower, then the burden can be higher without exceeding the output voltage limit if a transformer can carry 2 ohms at 50 amperes and deliver 200 volts. However, if the burden is 1 ohm at 200 amperes, it will not work since the internal impedance will be significant in relation to the 1 ohm burden. Figure 11: Accuracy Standard Chart for Class C Current Transformers Secondary Terminal Volts 800 700 600 500 400 300 200 100 0 C800 8 Ohm C400 4 Ohm 2 Ohm C200 1 Ohm C100 10 20 30 40 50 60 70 80 90 100 Secondary Amps 15
Insulation Systems Partial Discharges in Transformer Insulation Partial discharges (PD) are minute electrical discharges that result from the electric field stresses imposed on any insulation system. As the name suggests, they do not cause a complete electrical breakdown of the insulation, so their short term effect is not catastrophic. Over the long term, if the electric field stresses are high, PD can slowly deteriorate the quality of the insulation. In solid insulation systems (such as in instrument transformers), PD can occur where a void or discontinuity in solid insulation is introduced Figure 12. Because of the difference between dielectric properties of a void (filled with air or gas) and solid material, the localized electrical stress in the void can be higher than in a solid. This will cause a void to break down although the voltage across the solid will remain (Figure 12). These localized void breakdowns, resulting in small, high frequency current impulses, can be detected by using sensitive instrumentation. Sophisticated, specialized analysis of PD patterns can then be used to gain insight to the nature of PD, its possible location, and mitigation. PD is measured in the pico Coulombs (pc) unit of electrical charge. After many years of deliberation, different standards for different electrical equipment (ANSI, IEC, IEEE) do not consistently agree on the allowable or maximum limits of PD. Manufacturers of ITs use different ways of minimizing or controlling the level of PD: Control manufacturing processes (casting, curing, temperatures, vacuum, viscosities, etc.) to minimize the introduction of voids Develop shielding materials and techniques to minimize electric field stress enhancement Fill voids with dielectric gas to lower the risk of void breakdown Use insulation materials with similar dielectric constants for the solid insulation system Figure 12: Partial Discharges Electrode Source of Voltage A ~ Solid Insulation Void Space Source of Voltage B ~ Applied Source Voltage Capacitance Representing Void Space A. A sample of solid insulation between electrodes, which forms a capacitor, can have a void as shown. C Current Voltage...... B. The electrically equivalent capacitance network. C. Applied voltage and the corresponding PD current caused by breaking down the void................................. Normal Charging Current 16
Overloading, Overheating and Aging All insulation materials are deteriorated by the combination of overheating and exposure to moisture, oxygen in air, and UV radiation in outdoor conditions. ABB insulation systems are designed to withstand degradation caused by all of these environmental factors. IEEE Standard C57.13 recognizes two classes of insulation insofar as resistance to temperature is concerned: (a) 55 C temperature rise and (b) 80 C rise, both over a 30 C daily average ambient temperature. These values are the average temperature rise of the winding (as measured by rise of resistance) during the temperature test at rated maximum continuous current. The Guides for Loading recognize that these temperatures can be considerably exceeded for short periods of time without causing excessive deterioration of the insulation (reference Section 10 of IEEE C57.13). Maintenance and Inspection Testing of Insulation Instrument transformer users routinely test new transformers, as well as transformers in service, to ensure their adequacy for service. It is rarely possible for the end user to run complete series of tests, but there are some things the user can do for reassurance. Measurement of the resistance of each winding to ground (when one winding is measured, ground all other winding terminals) with a megger will indicate if something has happened to reduce the resistance values. Such an incident is most improbable on encapsulated transformers. All ABB insulated current and voltage transformers should have typical readings from the high voltage winding to the low voltage winding, and ground above 1 Megohm per volt at 25 C. Insulation resistance should be measured at ambient temperature (not over 30 C) because it decreases rapidly at higher temperatures. 17
Voltage Ratings Current transformers are always rated at the line-to-line voltage of the three-phase system on which they will operate. A 13.8 kv current transformer, for example, is designed for use on a 13.8 kv three-phase system. The actual voltage from the current transformer primary winding to ground is only 13.8/e3 or 7.9 kv as shown in Figure 13. Voltage Ratings for Current Transformers In the Transformer Standards IEEE C57.13, an insulation class, which has the appearance of a system voltage rating, is associated with each of the standard arrays of dielectric tests (60 Hertz and impulse voltage). It has become standard practice to apply transformers on systems with actual voltage higher than the insulation class value. This is done on the basis that if the power system is designed such (grounded, usually) that the line-to-ground voltage can never be more than 70% or 80% of the line-to-line value, lower-voltage-rated lightning arresters can be used and the insulation is protected from all the higher voltages to which it might otherwise be subjected. The transformer test voltages consist of a full wave impulse, chopped waves, 60 Hertz applied, and induced voltage test according to the schedule outlines in IEEE C57.13 Figure 13: Equivalent System Diagram Line Capacitance 13.8 kv 13.8/ 3 kv 13.8/ 3 kv 13.8 kv 13.8/ 3 kv 13.8 kv Whether or not this neutral is actually connected to ground, the natural symmetry of the circuit and the equal line capacitors to ground will cause the neutral to assume ground potential. 18
Voltage Ratings for Voltage Transformers The voltage which may be applied to a voltage transformer is limited not only by the permissible voltage to ground (as it is in a current transformer) but by the insulation between turns, between layers, and coil sections. It is also limited by the ability of the core to carry enough magnetic flux to induce the voltage. The voltage transformer is somewhat different from the distribution transformer, and certainly from the power transformer, in having a very limited capacity to store energy. Over Voltage Limits IEEE C57.13 recognizes five groups of voltage transformers for different capabilities and connections. These groups are explained by Figures 14 through 18. All transformers, according to IEEE C57.13, are capable of operating continuously, and of maintaining their accuracy at 110% of rated voltage. As indicated in the figures, Group 2 transformers need do no more than this, and Group 1 transformers must be able to operate (but not necessarily maintain accuracy) at 125% voltage during emergencies, while Group 3 and 4 transformers must be able to operate for one minute at line-to-line voltage, which is e3 times their rating. Group 5 must be able to operate (but not necessarily maintain accuracy) at 140% voltage for one minute. Figure 14: Application Limitation - Group 1 Voltage Transformers Grounded or Ungrounded System Primary Winding H1 H2 H2 H1 System Line-to-Line Voltage Transformer Rated Voltage H1 H2 Normal Operation Transformer Primaries in Delta Fault to Ground may be permitted to exist as far as the transformers are concerned. H1 Grounded or Ungrounded System H1 H2 H2 H2 System Line-to-Line Voltage 3 x Transformer Rated Voltage H1 Normal Operation Transformer Primaries in Wye Transformer Neutral Grounded. Fault to Ground must be cleared quickly because Group 1 transformers are good for only 125% voltage for emergency service. 19
Figure 15: Application Limitation - Group 2 Voltage Transformers Grounded or Ungrounded System H1 Primary Winding H2 H2 H1 System Line-to-Line Voltage Transformer Rated Voltage H1 H2 Normal Operation Transformer Primaries in Delta Fault to Ground must be cleared fairly quickly because the insulation at the terminals of Group 2 transformers is not good for rated winding voltage to ground continuously. H1 Grounded or Ungrounded System H1 H2 H2 H2 System Line-to-Line Voltage Transformer Rated Voltage H1 Normal operation transformer primaries in Wye Rated at Voltage on each winding 3 Transformer Neutral Grounded Fault to Ground may be permitted to exist as far as the transformers are concerned. Figure 16: Application Limitation - Group 3 Voltage Transformers System Line-to-Line Voltage Grounded System 3 x Transformer Rated Voltage These terminals are insulated only for 5 kv class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 14,400 for 24,940 Ground Y. Fault to Ground must be cleared within one minute because Group 3 transformers are good for more than 110% voltage for only one minute: 173% for 1 minute for ratings thru 92,000 for 161,000 Ground Y, 140% for 1 minute for ratings above this. These transformers have two secondary windings, one with a rating of 115 volts (except for the lowest rated 14400 for 25000 ground wye which is rated at 120 volts) and another secondary winding rated at approximately 115 V/e3. The value is not always exactly 115 V/e3 because, for simplicity, the primary/secondary voltage ratio is adjusted to a round number. 20
Figure 17: Application Limitation - Group 4 Voltage Transformers System Line-to-Line Voltage Grounded System 3 x Transformer Rated Voltage These terminals are insulated only for 1.2 kv class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 7,200 for 12,470 Ground Y. Fault to Ground must be cleared within one minute because Group 4 transformers are good for 125% voltage for only one minute. Figure 18: Application Limitation - Group 5 Voltage Transformers System Line-to-Line Voltage Grounded System 3 x Transformer Rated Voltage These terminals are insulated only for 1.2 kv class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 14,400 for 24,940 Ground Y. Fault to Ground must be cleared within one minute because Group 5 transformers are good for 140% voltage for only one minute. 21
Power Quality Power quality is an important consideration in designing any power system. Distribution systems are especially vulnerable to power quality problems. These problems should be taken into account when selecting and purchasing equipment. Distances for power delivery, density of loads, and customer concentrations are among the aspects that differ between the US ANSI market and the European IEC market. Power quality problems include: voltage sags and dips momentary interruptions (flicker) harmonics (harmonic current and/or harmonic voltages) Voltage Sags and Dips Voltage sags and dips are associated with switching or fault events in the power system that cause voltages on adjacent or neighboring circuits to partially collapse. These events can last from a few milliseconds to more than a second. Momentary Interruptions Momentary interruptions are the same type of power system events as voltage sags and dips, but are caused by lightning and other transients. The primary difference is that momentary interruptions occur primarily in the circuits directly involved in the event rather than in an adjacent circuit. Momentary interruptions are more severe power quality problems than voltage sags and dips. Figures 19A & 19B show statistics of voltage sags and momentary interruptions in a typical ANSI power system. Figures 19A and 19B: Distribution of RMS Events Interruption (V< 0.1) 9% 0.1< V< 0.5 10% Three Phases 13% Two Phases 19% 0.5<V<0.9 81% Single Phases 68% 19A. Pie-chart showing the statistics of voltage sags and interruptions. Note most events (81%) are sags (between 0.5 pu and 0.9 pu). 19B. Pie-chart showing the statistics of all power quality event types. Note most events (68%) are single phase. 22
Harmonics Harmonics are caused by non-linear loads. The nature of nonlinear power equipment causes a perfectly sinusoidal voltage waveform to result in currents containing other frequencies. A sinusoidal current can cause the generation of non-sinusoidal voltages. Non-linear power equipment includes saturated transformers, motors, and generators, but is primarily associated with power electronics. The act of triggering or switching a Silicon Controlled Rectifier (SCR), a diode, an Insulated Gate Bipolar Transistor (IGBT), or a Gate Turn-Off device (GTO), is in principle a non-linear operation. Power electronics devices such as Adjustable Speed Drives (ASDs), or Variable Speed Drives (VSDs) can exhibit a high level of harmonics, often more than 100% Total Harmonic Distortion (THD). In the case of ASDs, this level of harmonics varies with the selected rpm of the motor, and mechanical load on the shaft. Harmonics are unwanted events in distribution systems. They can cause excessive heating and damage to neutral connections and cables, and can saturate instrument transformers. Harmonics may also precipitate from the original location of the non-linear equipment to other locations like feeders and loads. They also may cause false tripping or malfunctioning of equipment, false readings from the CTs and VTs especially sensitive relays, computer loads, other ASDs, and Programmable Logic Controllers (PLCs). There are a variety of techniques to mitigate and control the level of harmonics, and industry standards regulating harmonics (see IEEE 519, Standard Practices and Requirements for Harmonic Control in Electrical Power Systems, or IEC 555/1000-3). Possible mitigation procedures include reconfiguring the system, re-sizing the cables or transformers to include additional loading due to harmonic currents, or installing filters and harmonic blockers. It is important to remember that these measures work for some, but not for all harmonics. Often a comprehensive study has to be performed to determine the level of harmonic currents, their impact on the power system, and possible suppression measures. In real systems, one disturbance in the system can cause a cascade of other disturbances. For example, a lightning surge (fast overvoltage transient) traveling along an overhead feeder line can cause a flashover and subsequent short circuit in a substation supplying an industrial plant, which in turn can cause abnormal acceleration of a local generator and so on. 23
Service Conditions Overload and Short Circuit Capability A voltage transformer is connected across the line or lineground, and is loaded to a greater or lesser degree depending on the number of devices connected in parallel at the secondary terminals (Figure 20). As the load is increased, the curves for ratio error and phase angle will show how the accuracy is affected. If accuracy is not important, the load can be increased to the thermal volt-ampere rating, the maximum which can be carried without overheating. Voltage transformers must be able to withstand an accidental short circuit for one second. A current transformer s primary is connected in series with the line and must carry whatever current flows in the line. The burden impedance connected to the secondary terminals affects the accuracy, as shown by the curves for ratio error and phase angle, but generally has no significant effect on temperature. Because watthour meters are usually capable of carrying at least 400% of rated current continuously, many of those current transformers used almost exclusively with watthour meters have been designed specifically to carry two to four times normal current. Also, because many of those current transformers are used almost exclusively in enclosed switchgear, they must be designed to operate in a high ambient temperature (55 C), they will carry 1.33 times normal current in a normal (30 C daily average) ambient temperature. The factor designating the continuous current capacity in terms of rated current at 30 C ambient is called thermal current rating factor. Standard values are 1.33, 1.5, 2.0, 3.0 and 4.0. The continuous thermal rating factor is based on 30 C average ambient temperature unless otherwise noted. The IEEE Guides for Loading recognize that transformer insulation can withstand a considerable degree of overheating for a short time without severe deterioration. For example, in the event of a line short circuit, the fault current may easily be fifty times the rated current of the current transformer, but will probably flow for not over one second. IEEE C57.13 permits a 250 C temperature for this very short time (compared to 95 C average continuous winding temperature). All current transformers are assigned a one-second thermal current limit which denotes how much current they will carry (usually denoted in terms of times normal rated current) for one second. For durations up to five seconds, current transformers will carry currents lower than this. The transformer will carry more current for a time less than one second according to the same rule, up to the mechanical current limit (which is also given for standard current transformers.) At this limit of current, the electro-mechanical forces tending to separate the primary and secondary coils, become high enough to damage the transformer. 24
The mechanical current limit is specified in terms of times normal rated current, as is the thermal one-second limit, but it is always assumed that the current may be fully offset initially. If it is known that the current cannot be initially fully offset, the current transformer will be able to withstand mechanically a larger RMS value of current, larger than its rated mechanical limit. The temperature at these high currents and short times (less than five seconds) cannot be measured, but is always calculated on the basis that all heat generated by the current is stored in the copper for the duration (not over five seconds) of the high current. Currents higher than the rated value, but less than the five second limit, can be determined by the rules set forth in the IEEE Guides for Loading. The calculation for any given transformer is lengthy, and as a general guide for standard transformers, the curves of Figure 20 can be used. Figure 20: Overload Capability of Current Transformers Times Maximum Continuous Thermal Current 20 15 10 9 8 7 6 5 4 3 2 1.5 1 Hours Minutes Seconds 1 2 3 5 7 10 20 30 50 70 100 Time in hours, minutes, and seconds following full load Recommended guide for short time loading of current transformers following rated load for 0.1% loss of life. Transformers so loaded will reach temperatures in excess of 55 C rise over ambient. Loading according to this curve is not safe if the ambient exceeds 30 C on the average, or if the overload occurs more often than once a day. Higher and Lower Ambient Temperatures At temperatures over the standard 30 C, daily average ambient temperature current transformers should be derated 1% for each degree over (up to 55 C ambient). At temperatures under 30 C, they may be uprated 0.75% for each degree (down to 0 C). Special application CTs are available where bus bar temperature is as high as 135 C (refer to Figure 21). 25
Figure 21: 55 C Rise Current Transformer Thermal Loading Guide 500 450 4.0 400 Percent of rated primary current 350 300 250 200 150 1.5 1.33 2.0 3.0 100 50 1.0 Curve designations are continuous thermal current rating factors at 30 C ambient air temperature. 0 0 10 20 30 40 50 60 Average Ambient Cooling air temperature for 24 hour period, degrees C. (Maximum ambient air temperature shall not exceed average by more than 10 C.) Open-Circuit Voltage in Current Transformers Figure 2 (page 5) shows that the line current must flow through the current transformer, and Figure 4 (page 8) shows that if the secondary circuit is accidentally opened, all the current will have to pass through the exciting current branches of the equivalent circuit. This will develop a high voltage across the exciting branch, which will appear as a high voltage at the secondary terminals. Because this voltage is limited by saturation of the core, the RMS value measured by a voltmeter may not appear to be dangerous. As the current cyclically passes through zero, the rate of change of flux at current zero is not limited by saturation, and is very high indeed. This induces extremely high peaks or pulses of voltage. These high peaks of voltage may not register on the conventional voltmeter, but they can break down insulation and are dangerous to personnel. Current transformers are insulated to withstand, for emergency operation, secondary peak voltages up to 3500 volts. This takes care of the smaller transformers with relay accuracy class under T200, but if open-circuit of larger transformers is probable, some protective circuit should be permanently connected to the secondary terminals. In general, open-circuit of the secondary terminals should be considered as a serious accident. The actual open-circuit voltage peak is difficult to measure accurately because it exists only as very short peaks. The method outlined in the Test Methods Section of IEEE C57.13 represents an excellent compromise between precision and feasibility. 26